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How the mind warps

When most people think about psychology, they think about psychopathology — about depression, psychosis, and multiple personality disorder — or about clinical interventions to help troubled couples and families stop fighting with one another. But psychology also deals with the fascinating, and often equally mysterious, workings of the normal brain.

We take the normal functioning of our brains for granted. It feels simple from the inside, like my eyes and ears projecting photos and videos straight back to the brain, which then quickly decides whether I am currently watching a Volkswagen ad during the Super Bowl or dancing with Jennifer Lopez’s beautiful cousin on a beach somewhere in the Caribbean.

But the reality is actually much different from that, and much more amazing. Your brain gets bombarded with a continual stream of sometimes confusing and misleading neural sparks, and has to engage in some amazing calculational feats to make sense of it all.

We can really appreciate how magnificent the process is by seeing what happens when it breaks down. If we examine puzzling visual illusions we can get an idea of how the brain normally does its work.

Here’s one of my favorite examples, called a complementary afterimage, in which your brain gets tricked into seeing something that simply isn’t there. I enlisted the talents of my film-producing son Dave to make a 1 minute video that’s worth checking out (click on the image to see it).

The afterimage is more than just a visual trick; it tells us several very important things about how our nervous system processes incoming information. For one thing, the mind is designed to detect change.

The next figure shows another example of the way our nervous system is sensitive to change. In the figure the patches of gray are all the same shade. Although I know that, my brain is still processing the patches on the black and blue backgrounds as brighter than those on the white and yellow backgrounds.

The brain's sensitivity to change explains why the house smells so wonderful when you walk in and mom is baking bread, and yet the smell seems to disappear in a few minutes. It helped our ancestors survive to be wired to detect change, rather than perseverating on the same information. “Gee, there’s a warm breeze rustling the leaves of the green tree in front of me; Hmmm, there’s still a warm breeze rustling the leaves of a green tree in front of me; Wow, there’s still a warm breeze rustling the leaves of a green tree in front of me!” Unless you accidentally ingested some hemp leaves with your salad: your mind quickly adapts to the old information, and only notices a change: as when the leaves move in a slightly different direction, and a large spotted cat climbs onto the branch above your head.

The flag afterimage also elucidates something else that is normally invisible. Your perception of any given color depends on the brain's upstream synthesis of incoming raw information from the sensory receptors in your eye's retina. The human eye has several different color processing systems.* Other animals, including your pet dog, are receptive to fewer dimensions of color, and would not see the negative afterimage (though Rover can smell in Technicolor, whereas our olfactory sense is the comparative equivalent of a grainy old black and white newsreel from 1920).

I’ll be covering some of my other favorite psychological phenomena in the next few weeks, and if I can talk my son into it, making a few more 1 minute videos that might be useful if you happen to teach an undergraduate class, or want to entertain your 8 year old nephew with how cool the mind is. In fact, after getting a lot of attention for my earlier controversial “adults-only” blogs (on topics like Why atheistic liberals are smarter and How women’s pheromones trigger economic riskiness in men), I’ve decided to try to talk about some aspects of psychology that I can actually show to my own 8 year old son. And when I started looking through my old introductory psychology textbook, I was reminded of how many fascinating and mysterious phenomena there are in the field. So stay tuned.

*My thanks to Tim Cannon and Martian Bachelor for pointing out problems with my previous wording of how this works. Though I must admit that I may still have the details wrong, given my status as a continuingly awed student rather than an expert on the psychology of vision. The lovely thing about being a researcher and college professor is that you actually get paid to be a lifelong learner.

Human color vision is trichromatic, not complementary. There are no yellow (sensitive) receptors. Rather, the opponent color model postulates the red and green receptor signals are added to synthesize a yellow channel. Exactly which colors appear as complements depends on the state of adaptation, i.e., which stimulus looks white (neutral). The system seems to have evolved to insure color constancy of objects under widely varying natural illumination conditions. Artifacts appear under highly contrived conditions.

Anyway, it's a good point to make that the senses generally are good at detecting changes against an adapted state rather than the absolute levels of those states, which more or less get calibrated out.